1. Field of the Invention
The present invention relates to the field of antenna receiving systems, and more particularly to a novel low profile ceramic choke for global positioning system antennas which is small in size, economical and may be readily mass produced.
2. Brief Description of the Prior Art
In the past, it has been the conventional practice to provide a filter reactor such as a coil or choke serving as an inductance in antenna systems. For example, a global positioning antenna is employed to receive signals which determine the location in longitude and latitude of the receiving system. In such highly precision navigational and surveying applications, the receiving antenna includes a choke slot ground plane incorporated with the receiving antenna in order to reduce phase error. The choke slot ground plane provides a highly capacitive surface that cannot support surface currents. This reduces re-radiation of these currents which in turn adds to the reduction of multi-path errors. Also, the choke slot ground plane allows the antenna to receive a cleaner signal and thus increase location accuracy.
Problems and difficulties have been encountered with conventional antenna chokes since the chokes are composed of metal and are bulky, large in size and, therefore, expensive to fabricate. Fabrication also includes or requires machining. A circular block of metal is machined so as to provide a plurality of concentric walls or ring-like partitions that are integral with a bottom support plate. The opposite side of the construction is open so that a plurality of alternate open spaces are defined between a plurality of different sized concentric thin cylindrical walls. In this manner, the air provides the dielectric between opposing surfaces of the concentric walls and an antenna is supported in the central area having the concentric walls of the choke surrounding the antenna. In order to provide for moisture or humidity protection, a plurality of holes is placed in the support plate so that drainage may be accommodated. Each circular gap or space between adjacent circular walls includes at least one, and preferably, multiple openings in the support plate to provide drainage. Furthermore, the conventional choke may further include a housing which is open in the center in order to expose the antenna while covering the circular choke.
Not only does the aforementioned conventional choke represent a bulky and expensive device to manufacture, but the collection and disposal of moisture within the air gaps of the choke adversely affects the efficiency and performance of the antenna. Also, the collection of moisture on the concentric and spaced-apart walls adversely affects heat dissipation and sometimes requires the expensive and costly necessity of hermetically sealing the metal housing. The ground plate may include holes for drainage in conventional chokes; however, these may clog and prevent drainage.
Therefore, a long-standing need has existed to provide a low profile ceramic choke for global positioning antenna application which is smaller in size than conventional chokes and which is lower cost and may readily be mass-produced without requiring machining and expensive materials.